The alignment or orientation of the various elements of a permanent magnet motor relative to each other, and especially with respect to the armature axis, becomes more and more important as such motors become more compact to match the configurations of household appliances, cordless portable power tools and other battery-powered products for home, vehicle and industrial use. Even relatively slight misalignment of the components can take its toll on such mechanical elements as motor bearings and drive members, which action can ultimately cut short the motor's useful life. Alignment is also critical to electrical parameters which affect the motor's efficiency. One of the major factors contributing to misalignment occurs as a result of the accumulation of variations in manufacturing tolerances in such elements as the end caps which rotatably support the armature, the permanent magnet array itself, and the ferromagnetic wrapper enclosing the magnet array and armature.
One approach to the problem has been to draw sheet metal into the shape of a single, closed-end cylindrical "can", which combines an end cap and the wrapper into a unitary structure. Here, the magnet elements are aligned relative to each other and to the armature by using the can and/or an outer surface of the end caps as reference surfaces. One resulting disadvantage is that additional alignment mechanisms, such as spring clips, tabs formed in the can inner surface, or adhesives, are required to maintain the predetermined orientation of the permanent magnets. This causes added manufacturing costs, either in parts or assembly time, or both. Another disadvantage directly attributable to locating the magnets using the can as a reference surface is that the "stack-up" (or accumulation) of variations in manufacturing tolerances is directed radially inwardly toward the armature. Consequently the air gap between the radially inner surfaces of a particular permanent magnet array and the armature will fluctuate with the tolerance stack-up, from a predetermined minimum to an excessive maximum. This means that many such motors will have more air gap than appropriate, and will suffer reduced efficiency. For battery-powered products, this is a significant problem.
Another disadvantage of this approach, as well as of any other system where the exterior configuration of the end caps are fixed, is that the electrically-powered device must to some extent be designed around the motor. This often requires, for example, that a motor having a fixed end cap configuration must be connected to the housing of the device using a separate mounting bracket and concomitant fasteners, all configured to match the shape of the end caps. Of course, tolerance stack-up can be minimized by using motor elements manufactured within very tight tolerances. On the other hand, forming ceramic or plastic permanent magnet segments within such tolerances will likewise result in higher manufacturing costs.
Another approach has been to form the wrapper into an open-ended tube using two discrete end caps, and to locate the permanent magnet segments on their respective inner surfaces using structural features formed on the end caps. This establishes the desired air gap, but the use of a wrapper having a fixed cross-sectional dimension still requires the use of additional elements to maintain the magnet orientation, including one or more of the following: spring clips, adhesives, tabs formed in the wrapper, or forming the wrapper into a particular polygonal cross-sectional configuration. Also, a characteristic of an open-ended wrapper constructed by forming a rectangular strip of metal into a cylinder is that often the parallel longitudinal edges of the rectangle become axially misaligned after forming. Here, even a relatively slight axial misalignment becomes important when the end caps are assembled to the wrapper, in that the end caps are skewed away from planes normal to the armature axis when they engage the misaligned edges. Consequently, the end cap bearing axes are skewed relative to the armature axis, and degradation of the end cap bearings is accelerated.
The increasing demand for permanent magnet motors magnifies the cost penalties associated with the above-noted solutions. The advent of the present invention provides a permanent magnet motor in which such costs have been taken out, while so enhancing the alignment of the respective motor elements as to extend motor life and improve motor efficiency.